[network_info_plus] Implement Submask, Broadcast and Gateway Address on MacOS

[miquelbeltran]

Currently the plugin returns empty strings, the implementation is missing.

The iOS implementation can be used for this.

[sameeerkashyap]

Hey @miquelbeltran, I was trying to figure out this issue, however I have very little native experience, could you give me a few pointers ?

[fabiancrx]

@miquelbeltran I think this issue can be closed since #571 was merged

[miquelbeltran]

thanks @fabiancrx !